The production and release of peptide growth factors by several transformed tissue culture lines developed from either human tumors or murine sarcoma virus (MSV)-transformed cells has been demonstrated. Serum free conditioned medium from these cells was used as the starting material for the characterization and isolation of these factors. Peptides have been purified from the serum free conditioned medium of a human fibrosarcoma line and a MSV-transformed murine line. The peptides from the fibrosarcoma bind to cell surface receptors for multiplication stimulating activity (MSA) and are capable of stimulating resting, serum depleted fibroblasts to divide. The peptide growth factors from the MSV-transformed cells are extremely potent mitogens and compete with epidermal growth factor (EGF) for the membrane EGF receptors. The peptides, which we call sarcoma growth factors (SGFs), were able to promote phenotypic changes in the morphology of monolayers of untransformed fibroblasts. Their morphology in the presence of SGF resembles that of MSV-transformed fibroblasts.